The developmental patterns of Substance P neurons in the central nervous system of rats has been examined by determining the SP concentration in various brain regions of prenatal and neonatal animals. The possible interaction of SP neuronal components with the endocrine system was investigated in pregnant, hypophysectomized and sphlanchnicectomized rats. Significant alteration in SP content was observed by neuroendocrine manipulation. In an effort to further probe the molecular mechanism of action for substance P an anti idiotypic antibody was raised against anti-Substance P immunoglobulin.